Generating a heart rate signal is simple enough, given a beat annotation file and tach, a program supplied as part of the WFDB Software Package. tach reads an annotation file, calculates an instantaneous heart rate for each cardiac cycle from the reciprocal of the interval between successive beat annotations, and then uniformily resamples this signal (by default, at 2 Hz). Referring to the WFDB Applications Guide, we find the command we need to generate a heart rate signal from `100s.qrs':
tach -r 100s -a qrs -f 0 -t 1:0
If this command is typed into the Analysis Commands window, it prints:
73.9726 73.9726 73.7238 . . . 75.7788 74.5703 73.8015These numbers are the calculated samples of the heart rate signal (the units are beats per minute).
We can generalize this command as:
tach -r $RECORD -a $ANNOTATOR -f $START -t $ENDwhere the parameters $RECORD, $ANNOTATOR, etc., are replaced by the appropriate values when the command is executed. This is the form of the command we should add to WAVE 's menu; by parameterizing the arguments in this way, we can use WAVE to select a record, annotator, and segment to be analyzed as for the other commands in the menu. To add this command to the menu, click on again. Position the cursor at the end of the menu file and add the line:
Heart rate<TAB>tach -r $RECORD -a $ANNOTATOR -f $START -t $ENDwhere <TAB> represents one or more TAB characters; at least one TAB must be present (spaces don't count). Be sure to end the line with RETURN. Save the menu (choose Save from the textedit window's menu), and quit from textedit. Now click on in the Analyze window. The Analyze window will disappear briefly, then reappear (possibly in a different location, depending on your window manager) with a new button. If you click on this button, WAVE executes the tach command as above.
You might prefer to look at heart rate as a signal rather than as a column of numbers. tach has the capability of generating an output signal file instead of text. To try this out, edit the menu file again, and add `-o hr_$RECORD' to the end of the entry. (You may split the command across two lines using a `' at the end of the first line if you wish.) Once again, save the menu file, quit from textedit, click on , and on . This time, there is no text output from tach; if you enter `hr_100s' in the Record field of the Load window, however, WAVE displays the calculated heart rate signal.
George B. Moody (email@example.com)